Dependencies and Task Graphs
Tasks can be linked together such that a dependent task does not start until its prerequisites are complete. This functionality enables the creation of "task graphs" containing multi-task dependency chains. For example, a task graph to build and test an application might start with a set of library build tasks (one for each platform), followed by application build tasks depending on those library builds, and culminating in test tasks that depend on the application build tasks.
Dependencies are defined by listing prerequisite taskIds in the
property of each task. When a new task is submitted to the Queue service, any
tasks listed as a dependency must have already been submitted to the Queue
requires task property controls whether the dependent task
should execute if any of its prerequisites fail.
A task can depend on itself -- this is called a "self-dependency". Such a task will not run automatically, but can be force-scheduled as described in Manipulating Tasks.